1. Field of the Invention
This invention relates to an adjustable vibration damper for motor vehicles or the like and, more specifically, to such an adjustable vibration damper which includes a damping force control by a valve device which selectively utilizes different damping elements which are capable of decompression and compression damping.
2. Description of the Prior Art
The prior art includes hydraulic, adjustable vibration dampers, whose damping force in the decompression and/or compression stage can be modified during operation (e.g. German Patent Publication Published for Opposition Purposes No. 12 42 945). By means of these devices, the damping force characteristic of the vibration damper can be adjusted either manually or by the action of one of the vehicle mechanisms, to current operating conditions such as the speed of the vehicle. A disadvantage of such dampers of the prior art is that, because of the design, the speed of the adjustments is very low, so that the reaction to oscillations of the wheel suspensions is very sluggish.
The prior art also includes fast-acting fluid control devices for vehicle suspension systems (e.g. German Laid Open Patent Appln. No 34 26 014). The rapid switching action disclosed therein makes it possible to react to movements of the vibration damper by means of a damping force adjustment. The object of such vibration dampers is to stabilize the vehicle superstructure and thus to keep its movements as small as possible. For this purpose, the vibration damper is configured so that it is possible to measure the movements of the body superstructure and the wheel suspension and, under such movement conditions, to select a large damping force so that the direction of the damping force produced is opposite to the movement of the vehicle superstructure or to select a small damping force, if the damping force and vehicle superstructure movement are in the same direction. In order to achieve such an objective, it is necessary to adjust the damping force at the instant the vibration damper changes its direction of movement. Because of a phase shift between the desired damping force and the actual value achieved, the effectiveness of the damping method on which the system is based is seriously compromised. To reduce the phase shift between the desired value and the actual value during dynamic oscillations to a tolerable level, the damping force must be set rapidly. Given the characteristic frequencies of the wheel suspensions of approximately 10-15 Hertz (for automobiles), there is only about 2-10 milliseconds available to measure the movement of the vehicle superstructure, measure the relative wheel movement, process the signals, trip the switching process and execute the switching process. In actual practice, it is difficult to achieve such switching times.
A further disadvantage of this prior art system is that, in order to recognize the current movement status, not only are measurement sensors for the movement of the vehicle necessary, but also sensors for the movement of each individual wheel. On account of the need to measure several values, both the time required for signal processing and the costs for the entire system are significantly increased.